This invention relates to apparatus for the production of fine powders from a liquid melt by gas atomization and solidification, and more particularly to a melt tube assembly for delivering a stream of high temperature melt to the atomization zone of such apparatus.
It is known to pass a stream of molten material, for example molten metal, through a nozzle or melt delivery tube, and to direct one or more high velocity fluid jets at the emerging stream to break up the stream into small globules, which are then solidified into particulates of varying sizes. Typical atomizing apparatus suitable for atomizing metals includes a heated crucible for melting or maintaining the melt temperature of the metal, a melt delivery tube for directing a stream of the melt to an atomizing zone below the crucible, and a gas nozzle to direct one or more streams or sheets of atomizing gas to impinge on the melt stream at the atomizing zone.
However, melt tube arrangements for atomizing molten metals for use in making powdered products have left much to be desired, particularly in the atomization of higher melting metals, i.e. those having melting points above 1000.degree. C., and especially of alloys having melting temperatures above 1200.degree. C. The major problems affecting known atomization apparatus and processes are: freeze-up (solidification of the melt) at the melt tube outlet, erosion of the melt tube, and melt tube breakage. In confined gas atomization systems, i.e. systems in which the gas nozzle closely surrounds the melt delivery tube, the outer surface of the tube is subject to severe cooling due to the proximity or actual impingement thereon of the atomizing gas, the temperature of which is greatly lowered by expansion as it exits the gas nozzle. In contrast, the inner surface of the tube is exposed to high temperature metal melts, some in excess of 1200.degree.-1500.degree. C. Thus, the melt delivery tube experiences severe thermal stress due to this drastic temperature differential between its inner and outer surfaces. Further, the inner surface of the melt delivery tube is subject to substantial erosive forces due to the flow of the melt aspirated therethrough, while the entire tube experiences severe mechanical shock or spring forces during the start-up of the high pressure gas flow.
The superimposition of these mechanical and thermal stresses generally leads to catastrophic failure of the system due to fracture of the melt delivery tube. The change in melt tube geometry and melt outlet position due to the fracture leads to backpressure conditions on the melt causing a cessation of melt flow and even the bubbling of atomization gas upward through the melt in the crucible. These problems have greatly increased the cost of operating such a confined system in a production environment where component reliability over an extended time is a necessity. This in turn has led to the underutilization of confined gas atomization in production processes and has led to increases in the cost of the metal powders produced thereby.